The Art of Test, Part 1: Boards, Subassemblies, & Products

Some people leave "test" as an afterthought, but test equipment design can be orders of magnitude more complex than the equipment it is testing.

Any manufactured product should be tested. Certainly, any product that I have designed that is going to be produced has a test procedure associated with it. The great advantage of testing products based on microcomputers (MCUs) is that they can be used to speed up, or even execute, the whole test.

I work for a small organization. As a result, I get involved in all aspects of a product, including the design of the production test. Often, the test equipment design is orders of magnitude more complex than the equipment it is testing and -- generally speaking -- I have more fun designing the test than the product itself. Having been with the same company (and nominally in the same position) for 20 years, I have seen many different approaches used. Although it is still evolving, I feel I have some insight into the process. My interpretation of testing includes calibration and downloading the program into the MCU.

Downloading the firmware
I know of no modern microcomputer or flash memory that cannot be programmed in-circuit. Although all MCUs have some means to connect a programmer to the product, or use some kind of bootloader, I have discovered that our subcontractors actually prefer to program the MCU before PCB insertion. This is because, if there is a problem with the MCU, they can return it.

One problem that "rides beneath the radar" is that of the connection to the board. You may be surprised to learn that the guaranteed number of insertions for a connector can be in the tens. I know I have tried flat cable connectors for emulators/JTAG etc. many hundreds of times; for production, however, perhaps you should consider spring-loaded contacts like those used on a bed-of-nails (maybe integrate the programming header into the bed of nails). While at a previous Design West conference, I discovered a company called Tag-Connect, which makes a header of these contacts on a flying lead.

The Rabbit module is programmed via a flat ribbon cable. We made a bed-of-nails to program it, allowing the subcontractor to return the module if it fails, and saving wear-and-tear on (and subsequent replacement of)the flat cable.(Click here to see a larger image.)

Product or subassembly
The device should be tested as close to completion as possible, but there are provisos. If there is some process to be performed before complete assembly, then obviously this should be undertaken as a subassembly, but it may require further testing when completed.

This actually depends on how you connect to the board and how your test procedure works. For a simple product with only a few inputs and outputs, for example, you can test by stimulating the inputs and monitoring the outputs. Quick-release terminals are best to allow quick connection to the device being tested, which is typically referred to as the device under test (DUT). Sometimes it pays to have a jig to speed this up. With sufficient design aforethought, it would be possible to use JTAG interface making for a much simpler interface.

A test fixture to allow rapid test connection and release of a finished product.(Click here to see a larger image.)

For more complex tests where you are using a bed-of-nails, then obviously you need the unpackaged device. There could be other reasons. For example, it could be much quicker to test the units in bulk as shown below.

With this product, it is possible to test eight units simultaneously. Connections to each module are run across the breakout tabs and brought to a header on the panel that allows a connection to a test fixture. In this instance, the connector at the top is connected to a bed of nails.(Click here to see a larger image.)

In my next column in this mini-series, I'll be considering the topic "Controlling the test." In the meantime, I welcome your comments and questions.

I must admit that our move pales in comparason to the ones you are tal;king about. Nevertheless there are many frustrations, especially if the logistics haven't been well thought out. In our case we moved from a location where we were renting space within another organization to one that we own. So issues like stocking the bathrooms, kitchen, picking up the garbage and snowplowing were all new to us and someone has to look after that, but who? That is to say nothing about the responsibilty for the layout, fitting existing furniture iton that space, redeploying partitions, storage space and on and on. It didn't help that the guy organizing this first had bypass surgery and then a detached retina slap dab in the middle of all the preparations.